Allow users to opt-out or select subset of warnings

[ManifoldFR]

Right now, compiler.cmake introduces the following warnings:

jrl-cmakemodules/compiler.cmake

Lines 18 to 36 in 2ede15d

	macro(_SETUP_PROJECT_WARNINGS)
	# -Wmissing-declarations is disabled for now as older GCC version does not
	# support it but CMake doest not check for the flag acceptance correctly.
	set(
	GNU_FLAGS
	-pedantic
	-Wno-long-long
	-Wall
	-Wextra
	-Wcast-align
	-Wcast-qual
	-Wformat
	-Wwrite-strings
	-Wconversion
	)
	if(NOT DEFINED CXX_DISABLE_WERROR)
	list(APPEND GNU_FLAGS -Werror)
	endif(NOT DEFINED CXX_DISABLE_WERROR)

In particular, -pedantic and -Wall introduce quite a lot of warnings. For interacting with some third-party header-only C/C++ libraries like magic_enum, -Wmissing-field-initializers and -Wconversion on GCC can get really spammy and annoying.

Another thing is that these flags are set globally, instead of on a per-target basis.

[ahoarau]

@ManifoldFR You can always use the SYSTEM keyword to remove some warnings from 3rdparty libraries.
On CMake 3.25 -> add_subdirectory(<3rdparty_dir> SYSTEM), also on FetchContent_Declare(... SYSTEM)

[ManifoldFR]

@ahoarau I agree and I do use that sometimes. The problem is when you're literally just copying the third-party header itself (without using a CMake listfile or anything).

[ahoarau]

Then you can create the magic enum target and declare its header SYSTEM:

add_library(magic_enum INTERFACE)
target_include_directories(magic_enum SYSTEM INTERFACE /the/path/to/header_dir)

And obviously link against it.

[ahoarau]

Or treat it like an imported lib:

add_library(magic_enum INTERFACE IMPORTED)
set_target_properties(magic_enum PROPERTIES 
                INTERFACE_SYSTEM_INCLUDE_DIRECTORIES /the/path/to/header_dir)